Electroluminescent devices using organic thin films, namely, organic electroluminescent devices each generally include a substrate bearing an anode, a cathode, and one or more organic layers including at least a light-emitting layer. The one or more organic layer are arranged between the two electrodes. Such organic layers may include a hole injection layer (anode buffer layer), a hole transport layer, a hole blocking layer, an electron transport layer, and an electron injection layer, in addition to a light-emitting layer. These layers are generally arranged or laminated between the anode and the cathode to constitute an organic electroluminescent device.
Organic electroluminescent devices have used fluorescent emission. As an attempt to raise luminous efficiency of the devices, it has also been examined to use phosphorescent emission instead of fluorescent emission. Sufficient luminous efficiency, however, has not yet been obtained even when phosphorescent emission is used.
Most of developed organic electroluminescent devices using phosphorescent molecules include materials containing carbazolyl group(s) as materials (host materials) for the light-emitting layer. For example, Non-patent Document 1 discloses, used as a host material, the following biphenyl derivative:

However, an organic electroluminescent device using the biphenyl derivative does not yield a satisfactorily high luminous efficiency, because the recombination of charge tends to occur unevenly in the vicinity of the cathode, and the device has poor balance in charge recombination.
Patent Document 1 mentioned below discloses an organic electroluminescent device using the following compound as a host material which enables a recombination region to concentrate in a light-emitting layer and has both hole transporting ability and electron transporting ability.

The device using the compound, however, shows light emission only under a high voltage and is insufficient in luminance and luminous efficiency. This is because the compound has only one pyridine ring, thereby shows a lowest unoccupied molecular orbital (LUMO) localized in the one pyridine ring, and shows a low electron transporting ability. In addition, the compound is insufficient in durability upon one-electron reduction.
Patent Document 2 proposes materials for use in organic electroluminescent devices typified by the following compounds:

These compounds, however, each have only one pyridine ring or pyrazine ring and are susceptible to improvements in electron transporting ability and durability upon one-electron reduction, as described above. In this connection, compounds having a pyrazine ring and/or a pyrimidine ring each containing two or more nitrogen atoms per one ring may be not suitable as host materials, because they may have a lower triplet excitation level and lower durability upon one-electron reduction than compounds having a pyridine ring.
Patent Documents 2 and 3 propose materials for use in organic electroluminescent devices typified by the following compounds:

These compounds are supposed to have somewhat improved electron transporting ability, because they each have two pyridine rings directly bound to each other (bipyridyl skeleton) and show somewhat delocalized LUMO. These compounds, however, are still insufficient in electron transporting ability. This is because all the carbon atoms at the ortho positions with respect to the carbon atom in one pyridine ring directly bound to another pyridine ring are each bound to a hydrogen atom or a substituent in these compounds. Accordingly, the bipyridyl skeletons of these compounds are poor in planarity due to steric hindrance, and the compounds have still insufficient delocalized LUMO. In addition, the compounds are insufficient in durability upon one-electron reduction and fail to achieve a practically sufficient driving lifetime. This is because the compounds are resistant to increase in planarity of the bipyridyl skeleton, but such a bipyridyl skeleton is to have increased planarity upon one-electron reduction.
Patent Documents 2 and 4 propose materials for use in organic electroluminescent devices typified by the following compounds:

Although these compounds are expected to have improved electron transporting ability and durability upon one-electron reduction, they are not well balanced in between hole transporting ability and electron transporting ability as materials for light-emitting layers of organic electroluminescent devices and are still susceptible to improvements as host materials.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-1972
Patent Document 2: PCT International Publication Number WO 03/078541
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-273190
Patent Document 4: PCT International Publication Number WO 03/080760
Non-patent Document 1: Appl. Phys. Lett., vol. 75, p. 4, 1999